Analytica Chimica Acta, 184 (1986) 213-218 Elsevier Science Publishers B.V., Amsterdam - Printed in ‘Ihe Netherlands

DETERMINATION OF ARSENIC BY HYDRIDE GENERATION WITH A LONG ABSORPTION CELL FOR ATOMIC ABSORPTION SPECTROMETRY

W. J. WANG, S. HANAMURA and J. D. WINEFORDNER*

Department of Chemistry, University of Florida, Gainesville, FL 32611 (U.S.A.)

(Received 21st October 1985)

SUMMARY

A method is described for the determination of arsenic involving hydride generation and atomic absorption spectrometry with an improved long graphite-tube furnace capable of considerably higher temperatures than the conventional quartz-tube heaters. Arsine is generated with sodium tetrahydroborate, held in a nitrogen-cooled trap and then swept with helium into an alumina tube (19 cm long) placed within the graphite furnace. The optimum conditions for determination of arsenic are given. The detection limit is 0.2 ng ml-’ with RSD of 2-3%. Results for various NBS Standard Reference Materials agreed well with expected values and were as follows: orchard leaves, 10 + 1 rg g-l; tomato leaves, 0.28 * 0.03 rg g“; bovine liver, 0.046 * 0.006 rg g-l.

Graphite electrothermal atomizers are commonly used as absorption cells in atomic absorption spectrometry. Most graphite furnaces are designed to vaporize and atomize the sample into a relatively small volume so as to im- prove sensitivity. L’vov [l] and Woodriff and co-workers [ 2, 31 have given quantitative relationships for absorption with respect to length, temperature, and pressure. These workers as well as others have shown that the overall sensitivity is proportional to the 3rd power of the length and that analyte loss by diffusion is less serious with long closed tubes than it is for tubes with openings near the center.

Ng and Caruso [4] recently reviewed electrothermal vaporization for sample introduction in atomic spectrometry and stressed the importance of independent control of vaporization and atomization which results in higher sensitivity and more efficient atomization of analyte in different sample types introduced into the atom reservoir. Hydride generation is one means of inde- pendent introduction of suitable analytes into furnaces, flames, and plasmas. Nakahara [ 51 has reviewed the principles and applications of hydride-genera- tion techniques in atomic absorption spectrometry. Improvements in detect- ability of hydride generation over direct introduction of sample solutions were obtained in most cases [ 6,7]. In this paper, a hydride-generation system coupled with a long-tube graphite furnace is used to quantify arsenic and is evaluated with respect to optimum conditions and quantitative applications.

0003-2670/86/$03.50 0-1986 Elsevier Science Publishers B.V.

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The long-tube furnace achieves a much higher temperature (>12OO”C) than is possible with conventional quartz vessels. Thus atomization is much more efficient for many elements.

EXPERIMENTAL

Apparatus A Hitachi Model 180-80 atomic absorption spectrometer equipped with a

tube graphite furnace (14 cm long), hollow-cathode lamp (Hamamatsu), and a Hitachi Model 056 strip-chart recorder were used. General aspects of the absorption cell are shown in Fig. 1.

The apparatus used for the generation of arsine is similar to the one de- scribed by Uthus et al. [ 91. Helium is used as the amine carrier because it has a lower boiling point than liquid nitrogen, which avoids clogging that would take place with argon. The line from the gas cylinder is split with a glass Y-joint into two lines provided with separate flowmeters. One line is con- nected to the left side inlet of the graphite tube to sweep the helium/amine mixture into the furnace; the other line is connected to the quartz tube as a sheath gas to minimize degradation of the graphite heater tube. Because of the outer diameter of the long tube furnace, the magnet of the Zeeman spec- trometer could not be used and so background correction was made sepa- rately from sample measurements. A new long tube furnace is now being designed for use with the Zeeman magnet to allow background correction.

Reagents A standard stock solution of arsenic was prepared by dissolving arsenic tri-

oxide (SRM 83b, National Bureau of Standards). High-purity hydrochloric

Fig. 1. Cross-sectional view of long absorption cell: (1) inner tube (i.d. 2.4 mm, o.d. 6.5 mm, length 19 cm, alumina); (2) quartz tube (o.d. 27 mm, length 14 cm); (3) heater tube (i.d. 6.5 mm, o.d. 9.5 mm, length 14 cm, graphite); (4) sample gas inlet tube (alu- mina); (5) argon carrier gas inlet tube (brass); (6) sample gas inlet tube holder (graphite); (7,8) graphite electrodes; (9, 10) terminals; (11) quartz window; (12) insulator; (13) elec- trode cooling water tube; (14) sheath gas inlet; (15) electrode assembly holder (brass); (16) electrode assembly holder binding bar (brass).

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and nitric acids were prepared by using the sub-boiling method [lo]. The aqueous 10% (w/v) sodium tetrahydroborate solution was prepared by dis- solving the sodium salt (A.C.S. grade, Fisher Scientific) in aqueous 0.5% (w/v) potassium hydroxide (86%, Mallinckrodt). This solution was filtered through No. 41 filter paper (Whatman) before use. An aqueous 20% (w/v) potassium iodide (A.C.S. grade, Fisher Scientific) solution was used. De- ionized water was used throughout.

Preparation of NBS Standard Reference Materials (SRM) Standard reference materials (NBS) were dried in a desiccator for 2 days

over phosphorus pentoxide. Approximately 250 mg of sample (orchard leaves, SRM 1571; tomato leaves, SRM 1573; bovine liver, SRM 1577) was weighed into a 300-ml round-bottom flask, and then 10 ml of high-purity nitric acid and 1 ml of sulfuric acid were added. The sample/acid mixture was digested with a condenser and a laboratory-constructed nitric acid reser- voir. The side-arm reservoir was used to collect the condensed nitric acid after the production of sulfur trioxide fumes; the flask was then cooled and the condensed nitric acid solution was returned to the sample mixture. The digesting procedure was repeated until the residual solution became clear. The solution was transferred to a teflon beaker and evaporated to white fumes. Finally, the digested solution was transferred to a 50-ml volumetric flask and diluted with deionized water to the mark. This sample was then used directly for arsenic measurements. Standard calibration solutions were prepared in loo-ml flasks from a lOOO-mg 1-l stock solution of arsenic con- taming 4% (v/v) hydrochloric acid.

Arsenic generation Figure 2 shows the block diagram of the amine generation system. The

l-ml sample was placed in the loo-ml round bottom flask with 1 ml of the iodide solution, 2 ml of water, and 1 ml of 6 M hydrochloric acid. The solu- tion was stirred, 1 ml of the tetrahydroborate solution was added, and the amine was collected by the liquid nitrogen trap; this process required about 6 min. Power was applied to the graphite furnace, which was adjusted to a constant temperature before the liquid nitrogen dewar flask was replaced by a beaker which was filled with water at room temperature.

RESULTS AND DISCUSSION

The optimum amounts and concentrations of hydrochloric acid, iodide and tetrahydroborate were selected from previous studies [9, 11, 121. The effects of the flow rates of argon and helium carrier gas on peak height are shown in Fig. 3. An argon flow rate of 120 ml min-’ and a helium flow rate of 55 ml min-’ gave maximum absorption for the samples used.

The amine signal for 1 ng of arsenic was shown to reach a plateau over the range 6-10 min trapping time, indicating complete generation, trapping, and

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Fig. 2. Block diagram of amine generation system: (A) helium; (B) generating flask; (C, D) dry ice/acetone cold traps to collect water; (E) CaCl, drying tube; (F) liquid nitro- gen cold trap; (G) argon.

060 A

050.

o 040.

z

2

B 030.

:: 4 020. 1./

010.

000 0 60 120 160 240

Argon flow rate (ml m&l

300

Fig. 3. Absorbance vs. flow rate: (A) argon for 10 ng As; (B) helium for 1 ng As.

0001 0 IO 20 30 40 50 60 70 60 90 I

Hehum flow rate (ml mm-’ 1

release of the amine. This result agreed with the results of Shaikh and Tallman [ 131. The trapping time limited the sampling rate of the method; in the present system, it is about 6 samples per hour.

With the long absorption cell, the limit of detection was 0.2 ng ml-’ (S/N = 3). The arsenic content of various NBS standards was also determined (Table 1). The values obtained for six determinations agreed well with the certified values. According to Kang and Valentine [ 141, residual sulfuric acid below 5% in the final biological sample solution did not interfere with the determination of arsenic as long as the amine was collected in a suitable trap and released rapidly for measurement.

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TABLE 1

Arsenic content of NBS Standard Reference Materials

SRM Arsenic content (rg g-l)

Present study* Certified value

1571: orchard leaves lO* 1 lOi 2 157 3 : tomato leaves 0.28 * 0.03 0.27 f 0.05 1577 : bovine liver 0.046 * 0.005 0.055 f 0.005

aMean and standard deviation for 6 determinations on dried samples.

Sturgeon and Berman [ 151 found that an increase of the partial pressure of oxygen in the graphite furnace suppressed the rate of condensed-phase thermal dissociation of analyte oxides, resulting in thermal shifting of the absorption pulses by the reaction M,Oy(s, 1) + xM(g) + y/202. The avail- ability of carbon atoms on graphite to adsorb oxygen resulted in an increase in the number of gas-phase analyte atoms. It was not sufficient to consider only the interaction between oxygen and the graphite surface, because graphite formed intercalation or lamellar compounds. Koreckova et al. [16] showed that 45% of 74As was left after pretreatment of the regular graphite tube at 1400°C for 90 s. Figures 4 and 5 show the effect of using the regular graphite tube alone and using an inner alumina tube with the graphite outer tube. It is clear that the inner alumina tube can prevent any interaction be- tween graphite and arsenic, so that the sensitivity is increased.

Figure 6 shows a plot of the absorption signal vs. furnace temperature; the furnace temperatures were measured with an optical pyrometer. It is apparent that the amine dissociates as the temperature increases above 1150-1200°C.

Temp PC)

Fig. 4. Effect of inner tube material on absorbance for cylinder arsine at a flow rate of 10 ml min-‘: (a) ASH, turned on; (b) ASH, turned off; (c, d) temperature increased from 800 to 930°C; (e, f) temperature increased from 930 to 1050°C. (A) Graphite inner sur- face; (B) graphite outer tube with alumina inner surface.

Fig. 5. Effect of inner tube material on absorbance from 10 ng As (1330°C, liquid nitro- gen trapping time 6 min). A and B as in Fig. 4.

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Temperature (IO3 “C)

Fig. 6. Absorbance vs. furnace temperature for 10 ng of arsenic.

Two atomization mechanisms have been proposed [8, 171 for amine; for both mechanisms, the appearance temperature was around 900°C. When tank amine was used, a plot similar to that in Fig. 6 was obtained. This implies that the atomization mechanism based on reduction of arsine with hydrogen to arsenic [ 171 is not likely, but the mechanism involving thermal dissociation of the As4 and As2 species [8] is a possibility.

This research was supported by EPA-R-810387-01-1.

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